1. Technical Field of the Invention
This invention relates to a drive shaft reduction gear, which transmits a rotation of a drive shaft to a driven body via a planetary gear mechanism.
2. Description of the Related Art
As a drive shaft reduction gear provided with a planetary gear mechanism which has been conventionally used, there has been proposed a structure of an independent suspension axle described in Japanese Patent Application Laid-Open No. 8-40095 or the like. The structure of the independent suspension axle described in the publication of the patent application mentioned above has a cross sectional structure shown in FIG. 6.
As shown in FIG. 6, in an axle housing 41 independently suspended by a vehicle body frame (not shown), a drive shaft 43, in which one end of the drive shaft is coupled to a universal joint 42, is supported at two points by a roller bearing 44 and a cylinder bush 45.
A sun gear shaft 47 having a sun gear 46 is fixed to an end portion of the drive shaft 43. A planetary gear 50 is engaged with the sun gear 46. The planetary gear 50 is pivoted to a planetary gear shaft 52 borne to a carrier 51. The planetary gear 50 is engaged with a ring gear 53 firmly fixed to the axle housing 41.
A ring gear hub 53a extending in a radial direction is attached to the ring gear 53. A shaft portion 53b extending in an axial direction is formed in the ring gear hub 53a. The shaft portion 53b is firmly fixed to an outer periphery of the axle housing 41 in accordance with a spline connection. The carrier 51 is connected to a wheel hub 54 rotatably attached to the axle housing 41.
The shaft portion 53b of the ring gear 53 is connected by spline to an outer peripheral portion of a shaft portion of the axle housing 41, and is non-rotatably attached to the axle housing 41. A movement regulation of the ring gear 53 in an axial direction is achieved by a locking piece 56 attached to an end portion of the axle housing 41. One bearing 55b bearing the wheel hub 54 is arranged in an outer peripheral portion of the shaft portion 53b of the ring gear 53.
The wheel hub 54 is borne by a pair of bearings 55a and 55b arranged in an outer peripheral portion of the axle housing 41 and an outer peripheral portion of the shaft portion 53b, respectively. A tire wheel (not shown) can be attached to the wheel hub 54. A rotation of the drive shaft 43 is reduced in speed by the planetary gear mechanism so as to be transmitted as a rotation of the wheel hub 54, and can rotate the tire wheel attached to the wheel hub 54 at a reduced speed.
The drive shaft 43 is supported at two points by the roller bearing 44 and the cylinder bush 45. Accordingly, even if a force in a radial direction is applied to the universal joint 42, the sun gear shaft 47 is not affected. Further, the sun gear shaft 47 is engaged by spline with the drive shaft 43 via a cylindrical joint 48. Therefore, the sun gear 46 is arranged in a floating state, and a tooth contact with the planetary gear 50 is kept good.
However, in the independent suspension axle structure described in Japanese Patent Application Laid-Open No. 8-40095, the bearing 55b is arranged in an outer peripheral portion of the shaft portion 53b. Accordingly, the structure is made such that the bearing 55b bearing the wheel hub 54 and the shaft portion 53b of the ring gear 53 are arranged between the axle housing 41 and the wheel hub 54. When the bearing 55b is exposed to a load applied from the wheel hub 54 so as to be deformed, a deformation load is applied also to the spline connection portion between the shaft portion 53b and the axle housing 41.
When the deformation load is applied to the spline connection portion between the shaft portion 53b and the axle housing 41, a micro slip due to a relative repeat is generated between two surfaces which are in contact with each other in the spline connection portion. An abrasion is generated between the two surfaces mentioned above due to the micro slip. In other words, a phenomenon that the abrasion is generated due to the micro slip, that is, a so-called fretting tends to be generated. Accordingly, there is a problem that durability is lowered in the spline portion due to the generation of the fretting.
Further, the wheel hub 54 is borne to the outer peripheral portion of the shaft portion 53b in the ring gear 53 via the bearing 55b. Accordingly, there is a problem that an outer peripheral diameter of the wheel hub 54 becomes large. Further, the structure is made such that the locking piece 56 achieving the movement regulation of the ring gear 53 in the axial direction is arranged in a space portion between the end portion of the axle housing 41 and the sun gear 46. Accordingly, it is impossible to make a thickness of the locking piece 56 sufficiently thick, and it is hard to sufficiently secure strength of the locking piece 56.